there are as many ways to construct a bus as there are builders.
that being said.....
i am from the cabinet type of people. all my bus is 3/4" plywood boxes screwed together with the help of a kreg jig.
all my plumbing and electrical are in the bus sidewalls, so no reason for any depth to the walls.
people frame like they do houses with 2x4's, some use metal 2bys like office buildings. some use metal and face frames of wood. i just used 3/4" ply and a kreg jig.
i posted a pic the other day of it
so i have it handy - Kreg jig\
and check out my build thread under my signature.